In an automated material handling system, items, such as boxes or cartons, are loaded onto pallets by a robot. For example, in automotive assembly plants, mixed-model assembly lines sequence subassemblies or parts for final assembly. Preferably, the parts are delivered in sequence starting from the suppliers for the assembly lines. The sequence can be disrupted as a result of missing parts, delays, rework, parallel workstations, and other factors. Such disruptions require time consuming re-sequencing of the parts.
One approach to item sequencing involves an automated storage and retrieval system (AS/RS), which uses computer-controlled systems to place items into and retrieve items from defined storage locations. Items are retrieved from the AS/RS (typically by cranes) and fed to the robotic pallet building workstations. To do this, a warehouse management system constructs a desired sequence and releases requests to the AS/RS in accordance with the sequence. However, if there are multiple cranes working in parallel, for example, the requests are not always fulfilled in the proper order. As a result, items often arrive out of sequence.
Another way to achieve the sequence of items required by the palletizing robot is to use a circular sortation conveyor. Items enter and loop around the conveyor until they are required at the workstation. Some disadvantages of this system include increased cycle time of requested items, inflexibility with respect to throughput capacity, and poor utilization of floor space.